1. Field of the Invention
This invention relates generally to flexible polyurethane foams. More particularly, this invention relates to thin, flexible polyurethane foams that are useful as backing materials in bandages.
2. Brief Description of the Related Art
Polyurethane foams are useful materials for a wide variety of applications, including as backing materials in bandages for wound care. Such foams may have densities in the range from about 10 to about 50 pounds per cubic foot (pcf) (about 160 to about 801 kilograms per cubic meter (kcm)). In order to act as an effective backing material, thin films of the polyurethane foams should have a good surface feel and be easily stretchable, yet also have high tensile and tear strength to prevent tearing during use. It is also important that the foams have a high water vapor transmission rate while at the same time a low liquid permeability to prevent leakage into or from the wound.
The preparation of flexible polyurethane compositions for use in wound care is generally known, as evidenced by the prior art. Hydrophilic, water-absorptive polyurethane materials specifically for the treatment of burns are disclosed in U.S. Pat. Nos. 3,978,266 and 4,233,969 to Lock. Other hydrophilic, water-absorptive materials are disclosed in U.S. Pat. No. 3,648,692 to Wheeler, U.S. Pat. No. 3,927,669 to Glatt, U.S. Pat. No. 4,550,126 to Lorenz, U.S. Pat. Nos. 4,655,210 and 4,733,659 to Edenbaum, et al., U.S. Pat. No. 5,844,013 to Kenndoff et al., and U.S. Pat. No. 5,292,777 to DesMarais et al. The purpose of such materials is primarily as sponges, medicament carriers, and surgical dressings for the absorption of liquid wound exudate.
Use of flexible polyurethane foams as backing layers has also been described. For example, U.S. Pat. No. 5,844,013 teaches the preparation of hydrophilic polyurethane gel foams, but with a polyurethane sheet as a backing. U.S. Pat. No. 4,738,257 to Meyer et al. discloses a highly elastic, porous polyurethane foam used as a backing material, which stretches upon absorption of water by a skin-contacting layer. Flexible polyurethane backing layers or tapes are also generally disclosed in U.S. Pat. No. 4,362,825 to Grabhoefer et al. and U.S. Pat. No. 3,665,918 to Lindquist et al. Grabhoefer et al discloses blown polyurethane foams manufactured using polyester polyols having OH numbers from 40 to 80 and molecular weights in the range from 1500 to 5000. The compositions are fairly specific, requiring, e.g., 1,4-butanediol. The foams of Lindquist require an extra manufacturing step, i.e., permanent compression of the foam to less than about 50 percent of its original thickness. Each of the foregoing is incorporated herein by reference.
While a number of these prior art foams meet at least some of the requirements for bandages, it has heretofore been difficult to impart good or excellent water vapor transmission rates and low water permeability to such foams, while at the same time maintaining the desired characteristics of easy stretchability, tensile and tear strength, and surface feel. It is especially difficult to obtain the requisite combination of high tensile and tear strengths while retaining easy stretchability and conformability. Because of these or other deficiencies such as ease of manufacture, these prior art foams are not ideal for use as bandage backing materials. Consequently, there remains a need for polyurethane foam compositions which have low liquid permeable but which transmit water vapor, and which still retain the required degree of easy stretchability, tensile strength, tear strength, and surface feel.
The above-discussed and other drawbacks and deficiencies of the prior art are overcome or alleviated by a composition for the formation of a soft, flexible polyurethane foam, comprising an active hydrogen-containing component, an organic polyisocyanate component having an isocyanate functionality of 2.00 to 2.25, a surfactant, and a catalyst component, wherein the OH number of the composition is from about 100 to about 180, and wherein formed foam has a density in the range from about 20 to about 5025 pcf (about 16 to about 801 kcm); the average cell size of the foam is about 20 to about 60 micrometers (microns); the typical cell openings are about 5 to about 25 microns, wherein the ratio between cell average diameter and cell opening average diameter is from about 3 to about 10; and the molecular weight between crosslinks is in the range from about 3000 to about 10,000. The individual cells are furthermore preferably substantially spherical.
Such foams are easily stretchable and conformable and have high water vapor transmission and low liquid permeability, while at the same time excellent tensile and tear strength, as well as acceptable surface feel. Because of the foregoing numerous features and advantages, the materials described are especially suitable for use as backing materials for bandages. The above-discussed and other features and advantages will be appreciated and understood by those skilled in the art from the following FIGURES and detailed description.